1. Field of the Invention
The present invention relates to a support base for a measuring cell.
2. Description of Related Art
Support bases of this type and associated measuring cells are used primarily with scales, but for the installation of machines.
A scale can be constructed as a platform scale or a tank scale and can include a weighing platform with at least three support bases and an equal number of measuring cells that respond to a pressure and to a twisting or shear load. Each support base is associated with a measuring cell.
The support bases and the measuring cells are arranged at a certain distance from each other inside the platform area of the scale. The measuring cells represent elements that connect the weighing platform with the support bases. In other words, the housing of the measuring cells is secured to the weighing platform, whereas the measuring arm is attached to the support base.
These scales are typically preassembled in the factory and are then adjusted at the installation site to conform to the local conditions.
The support bases are designed to receive the load to be measured and to exert a matching counter force, without substantially affecting the measuring cell. The support bases must therefore be articulated and able to support a large force. The height of the support base must also be adjustable.
Several types of support bases are known in the art. For example, European Patent No. 319 411 describes a support base with a joint consisting of a load pin that is under pressure and has a pressure head and a guided ball. The pressure head and the ball are clamped without clearance in a flexible rubber or plastic body. The ball is supported by the bottom surface of the base plate and the rubber or plastic body is supported by an outside wall of the base plate.
The joint described in the European Patent, however, is only able to transmit small forces because the ball contacts the planar surfaces the pressure head and the base plate, respectively, only at a single point and can therefore cause an indentation in the material. Even the selection of an extremely hard material cannot overcome this disadvantage. It is possible to increase the effective contact area by increasing the diameter of the ball. This approach, however, also increases the overall height of the support base and the scale which is a further disadvantage.
Another disadvantage is that since the connection between the pressure head and the rubber or plastic body does not provide any play, a displacement of the load pin distorts the rubber or plastic which, in turn, may affect the measurement process and misstate the results.
German Patent No. 83 03 337.8 U1 describes a joint having a ball that contacts an upper pressure plate and a lower pressure plate. Both pressure plates include a concave pressure surface facing the ball.
The concave shape of the surface areas slightly increases the contact area and thereby also the load bearing capacity. However the large diameter of the ball requires a relatively large support base.
Furthermore, if the load and the weighing platform move horizontally, then the ball may disadvantageously roll off and cause the pressure plates to come apart due to the concave shape of their surface area, thereby producing distortion and measurement errors.
German Patent No. 92 13 163.8 U1 describes a joint that includes a load pin under tensile load and is supported on both sides by respective bearings. Each of the bearings is formed by a spherical disk and a ball socket having contact surfaces of matching shape and size. This joint also produces measurement errors if the individual force transmitting elements became vertically misaligned. If the weighing platform is subjected to a lateral load or if the base plate is inclined, then the spherical disk and the ball socket would have to be able to slide relative to each other. This relative sliding motion of the components which is required for relative movement and displacement, requires adequate play between the two components that may noticeably distort the measurement results. The sliding friction also causes excessive wear of the components. Moreover, only small forces can be transmitted since the load is carried only by the screw connection located on both sides of the load pin.
Swiss Patent No. 0 250 939 discloses a stand and safety support device for truck scales wherein the support members are in the form of articulated columns. The support members have spherical ends and flat support surfaces.
German Patent No. 94 13 947.4 also describes a support base for weighing cells supported by a floor. The support base includes a pressure block with a flat pressure surface and a pressure element with a spherical pressure surface. Both pressure elements are pressed into a resilient rubber body.
European Patent No. 0 361 518 describes a platform scale with a support strut positioned inside a through hold located in the force measuring cell. A counter plate is arranged on the support strut, with an annular gap interposed between the support strut and the counter plate.
It is therefore an object of the present invention to develop a support base of the aforedescribed type that provides a joint with a large effective contact area, wherein a displacement of the joint does not require play and does not change the height of the support base.